In such sort of an electric power transmission device, a primary side power transmission coil and a secondary side power receiving coil are magnetically coupled and electric power is transmitted from the primary side power transmission coil to the secondary side power receiving coil via non-contact and out of touch. As described above, there is a large problem in that the electric power transmission device utilizing electromagnetic induction must improve electric power transmission efficiency and transmission electric power and reduce electric power consumption. Techniques to solve such problems have been proposed. For example, International Publication No. 98/34319 discloses a technique in which a primary coil and a secondary coil use a cored coil and a capacitor is connected to the secondary coil in parallel to form a parallel resonance circuit; and since a frequency higher than a frequency of oscillation signal of the primary coil side is chosen as a resonant frequency of the secondary coil side, a capacitance can be reduced and a coupling coefficient between the primary coil and the secondary coil can be apparently increased, and therefore electric power transmission efficiency can be improved. In addition, there is disclosed another technique in which the presence or absence of a load is indirectly detected by a parameter variation in current or voltage generated in the primary coil; and electric power is continuously supplied to the primary coil in a complete operation mode when loading, and electric power is intermittently supplied to the primary coil in an intermittent operation mode for each predetermined timing during predetermined time when no-loading, thereby reducing electric power consumption.